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| United States Patent |
5,354,724
|
|
Hoffmann
, et al.
|
October 11, 1994
|
Heat sensitive recording materials with polymer enrobed sensitizer
Abstract
Recording materials which contain one or more optionally polymer-enrobed
dye precursors, one or more optionally polymer-enrobed developers and one
or more polymer-enrobed sensitizers are described.
| Inventors:
|
Hoffmann; Dietrich (Roedersheim-Gronau, DE);
Schuler; Bernhard (Mannheim, DE)
|
| Assignee:
|
BASF Aktiengesellschaft (Ludwigshafen, DE)
|
| Appl. No.:
|
099008 |
| Filed:
|
July 29, 1993 |
Foreign Application Priority Data
| Current U.S. Class: |
503/209; 503/215 |
| Intern'l Class: |
B41M 005/26 |
| Field of Search: |
503/209,215,208
|
References Cited
U.S. Patent Documents
| 4406816 | Sep., 1983 | Sliwka | 521/69.
|
| 4520376 | May., 1985 | Morishita et al. | 346/204.
|
| 4749679 | Jun., 1988 | Yoshida et al. | 503/208.
|
| Foreign Patent Documents |
| 3512565 | Oct., 1986 | DE.
| |
Primary Examiner: Schwartz; Pamela R.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt
Claims
We claim:
1. Heat-sensitive recording materials comprising one or more optionally
polymer encapsulated dye precursors, one or more optionally polymer
encapsulated developers and one or more polymer encapsulated sensitizers,
wherein said one or more sensitizers are encapsulated separately from said
one or more dye precursors and said one or more developers and wherein
said sensitizers are monohydric or polyhydric aliphatic C.sub.6 -C.sub.34
-alcohols or mixtures thereof.
2. Heat-sensitive recording materials as claimed in claim 1, wherein the
polymer around the sensitizer consists essentially of a urea/formaldehyde
resin, a urea/melamine/formaldehyde resin, a melamine/formaldehyde resin,
a polyurethane, a polyurea, a polyamide, a polyester, a polycarbonate, a
polystyrene, a styrene/acrylate copolymer, a styrene/methacrylate
copolymer, a polyacrylate, a polymethacrylate, gelatin or combinations
thereof.
3. Heat-sensitive recording materials as claimed in claim 1, wherein the
polymer around the sensitizer consists essentially of a
melamine/formaldehyde resin,a urea/melamine-formaldehyde resin, a
urea/formaldehyde resin, a polyacrylate or a polymethacrylate.
4. Heat-sensitive recording materials as claimed in claim 1, wherein the
polymer around the sensitizer was formed in the presence of
2-acrylamidopropanesulfonic acid homopolymers or copolymers as protective
colloids.
5. Heat-sensitive recording materials as claimed in claim 1, wherein the
dye precursor has been provided with a first microcapsule comprising a
water-soluble, nonionic polymer and a second microcapsule comprising a
crosslinked polymer.
6. Heat-sensitive recording materials as claimed in claim 1, wherein the
developer has been provided with a first microcapsule comprising a
water-soluble, nonionic polymer and a second microcapsule comprising a
crosslinked polymer.
Description
The present invention relates to heat sensitive recording materials
comprising one or more optionally polymer enrobed dye precursors, one or
more optionally polymer enrobed dye developers and one or more polymer
enrobed sensitizers.
Heat sensitive recording materials, also known as thermal papers, are
widely used for printing and copying. They require fast reacting systems
notable not only for a bright, high contrast appearance of the script but
also for the long life of the image obtained. No graying of the
ready-produced paper must take place. Compared with other recording
techniques the advantages include the elimination of a development
process, the price, the quietness of the reproduction equipment, and the
intensity of the print.
The thermally reactive components present in thermal papers are a colorless
or substantially colorless dye precursor and a developer, and are both
solid at room temperature. Suitable dye precursors include not only leuco
dyes, which can be converted into the colored form by oxidation, but also
diazo components as precursors of azo dyes. In the first case, the
developer will be an electron acceptor, while in the second case it will
be a coupling component and, if necessary, a basic substance to speed up
the coupling reaction. Thermal papers are produced by applying the dye
precursor and the developer to a base paper in a single- or multi-stage
coating process together with binders and, if desired, pigments. Color
development is effected by briefly heating the paper locally with a
thermal printing head (thermal pulse). The reactive components melt to
some extent and react to form the dye and hence the color. To reduce the
necessary temperature and/or duration of the thermal pulse and hence to
increase the printing speed, it is customary to add a sensitizer which
itself is likewise solid at room temperature but lowers the melting points
of the color-forming components. The prerequisite for the use of a
sensitizer is that it should not lead to premature development either in
the production or in the storage of the paper coating compositions.
US-A-4 520 376 describes the enrobing of dye precursor, developer and waxy
sensitizer with a thermally stable microcapsule within which color
development takes place on heating. First the dye precursor and the
developer are each melted together with the sensitizer, emulsified, and
encapsulated together with the further addition of sensitizer. In one
version of this process the two mixtures are each first provided with an
extremely thin, thermally labile pseudo-microcapsule and only then
encapsulated together normally with further sensitizer. The microcapsules
obtained are then applied to the paper with a binder.
US-A-4 749 679 describes the encapsulation of the dye precursor. For this
the dye precursor first has to be dissolved in a suitable solvent and then
encapsulated in an aqueous phase. The color developer, optionally a
substance to speed up the color-forming reaction, and the aromatic
alcohol, carbamate and sulfonamide sensitizers are each dispersed in an
aqueous organic phase by milling. The dispersions obtained and the
microencapsulated dye-precursor are applied to the paper together.
The disadvantage of these processes is that, first, the choice of
sensitizer is limited to substances which do not cause premature
development, some of which are costly, and/or, secondly, a large number of
operations is required.
DE-A-3 512 565 discloses the encapsulation of waxy substances for preparing
spacers for carbonless copy papers.
It is an object of the present invention to provide heat-sensitive
recording materials which have good application properties and are
inexpensive to prepare.
We have found that this object is achieved by heat-sensitive recording
materials comprising one or more optionally polymer enrobed dye
precursors, one or more optionally polymer enrobed developers and one or
more polymer enrobed sensitizers.
The sensitizers used in the recording materials of the invention are rather
hydrophobic, only sparingly water soluble substances which are solid at
room temperature, which have a melting point of in general
35.degree.-150.degree. C., preferably 50.degree.-100.degree. C., and which
mixed with dye precursors and/or developers lower the melting points
thereof.
Suitable sensitizers are for example the waxy substances mentioned for that
purpose in US-A-4 520 376, e.g. vegetable waxes which are substantially
free of high molecular weight fatty acids, such as candelilla or carnauba
wax, hydrocarbon waxes such as paraffins, ozokerite or microcrystalline
waves, or else waxy polymers, such as polyethylene or polyvinyl ether
waxes or mixtures thereof.
The aromatic, in particular phenolic, sensitizers mentioned in US-A-4 749
679 can likewise be used.
However, particularly preferred sensitizers are monohydric or polyhydric,
in general monohydric, dihydric or trihydric, preferably monohydric or
dihydric, aliphatic C.sub.6 -C.sub.34 -alcohols such as hexanediol,
octanediol, decanediol, myristyl alcohol, tetradecanediol, eicosanol,
myricyl alcohol and in particular cetyl and stearyl alcohol and mixtures
thereof.
The particularly preferred alcoholic sensitizers are inexpensive but highly
effective substances which have a dissolving (melting) effect on the
precursor and/or on the developer even at room temperature.
In the thermal papers of the invention, the sensitizer is used in a polymer
enrobed form. For this reason there is no restriction on the choice of
suitable compounds, and the advantageous aliphatic alcohols can be used
without any problems.
A polymeric robe material for the sensitizer can be made not only of a
thermoplastic polymer but also of a thermoset polymer. The enrobed
sensitizer melts on heating and thus will destroy even a thermally stable
robe.
Suitable are the usual encapsulation polymers mentioned for example in
DE-A-3 512 565 such as polyurethanes, polyureas, polyamides, polyesters,
polycarbonates or polystyrenes, styrene/acrylate copolymers,
styrene/methacrylate copolymers, polyacrylates, polymethacrylates,
formaldehyde resins such as urea/formaldehyde, urea/melamine/formaldehyde
and melamine/formaldehyde resins and gelatin and also combinations
thereof.
Of particular suitability besides polyacrylates and polymethacrylates are
urea/melamine/formaldehyde resins, urea/formaldehyde resins and in
particular melamine/formaldehyde resins.
The enrobing of the sensitizer according to the invention can be carried
out in a conventional manner. It is customary to work at temperatures
above the melting point of the sensitizer. If necessary, a pressure vessel
has to be used.
Examples of suitable processes are: gelatin coacervation, interface
polycondensation to form polyesters or polyamides, interface polyaddition
to form polyureas or polyurethanes, free radical interface polymerisation
to form polyacrylates or polymethacrylates, the deposition of polymer
films by precipitation from polymer solutions, in particular from
poly-acrylates or polymethacrylates, and in particular the homo- and
co-condensation of urea and/or melamine with formaldehyde.
Particular preference is given to enrobing by polycondensation of melamine
with formaldehyde or of precondensates based on melamine/formaldehyde, in
particular methylolated and partially etherified, especially
methanol-etherified, precondensates.
Enrobing is advantageously carried out in the presence of protective
colloids, i.e. water soluble polymers which will stabilize the capsule
dispersions being formed.
Suitable are the known% polymers used for this purpose, e.g. polyvinyl
alcohol, salts of polyacrylic acid, copolymers of polymerizable di- or
higher carboxylic acids with vinyl isobutyl ether, ethylene and/or
(meth)acrylic esters, cellulose derivatives and in particular homo- or
copolymers of sulfo-containing monomers.
Examples of the last-mentioned preferred polymers are the homopolymers,
known from EP-A-26 914, of sulfoethyl (meth)acrylate, sulfopropyl
(meth)acrylate, maleimide-N-ethanesulfonic acid and in particular
2-acrylamido-2-methylpropanesulfonic acid and also the copolymers of these
monomers. Particular preference is given to
poly(2-acrylamido-2-methylpropanesulfonic acid).
Preference is further given to the copolymers of the abovementioned
monomers--likewise mentioned in EP-A-26 914--with C.sub.1 -C.sub.3 -alkyl
acrylates, hydroxy-C.sub.2 -C.sub.4 alkyl acrylates and/or
N-vinylpyrrolidone.
Particularly preferred copolymers are the copolymers described in the
earlier German Patent Application P 42 09 632.4 of
a) from 20 to 90% by weight, preferably from 40 to 75% by weight, of
2-acryla/nido-2-methylpropane-sulfonic acid, sulfoethyl or sulfopropyl
(meth)-acrylate or vinylsulfonic acid or salts thereof,
b) from 0 to 50% by weight of a vinylic acid, preferably from 20 to 40% by
weight of (meth)acrylic acid,
c) from 0 to 70% by weight, preferably from 10 to 50% by weight, of methyl
or ethyl (meth)acrylate, C.sub.2 -C.sub.4 -hydroxyalkyl acrylate or
N-vinylpyrrolidone, and
d) from 0.1 to 10% by weight, preferably from 0.5 to 5% by weight, of
styrene or C.sub.4 -C.sub.18 -alkyl (meth)-acrylate.
These copolymers are preferably prepared by a conventional free radical
polymerization in an aqueous phase.
Viscosities of the polymers of the invention in a 20% strength by weight
aqueous solution or dispersion at room temperature and at a shear rate of
489 s.sup.-1 are in general from 5 to 5000 mPa.multidot.s, preferably from
100 to 2000 mPa.multidot.s, particularly preferably from 250 to 1500
mPa.multidot.s.
With melamine/formaldehyde resin the sensitizer is for example in general
enrobed by heating the sensitizer in an aqueous phase to above its melting
point and dispersing it finely to a particle size of from 1 to 100 .mu.m
using a high performance disperser in the presence of one of the
protective colloids mentioned. The aqueous solution of
melamine/formaldehyde precondensate is then added over 1 to 240 min at a
temperature of in general 35.degree. to 150 .degree. C., preferably
35.degree. to 90 .degree. C., and at a pH of usually 3.0 to 7.0, in
particular 3.5 to 5.5, set with an inorganic or organic acid (e.g.
sulfuric acid, hydrochloric acid, phosphoric acid, formic acid, acetic
acid). After further heating at 35.degree. to 100 .degree. C., for 0.5 to
5 h, the dispersion is cooled down and neutralized with an inorganic or
organic base (sodium hydroxide, potassium hydroxide, calcium hydroxide,
ammonia, alcohol a/nines such as di- and triethanolamine).
The resulting dispersion of polymer-enrobed sensitizer can advantageously
be used directly in a coating composition for thermal papers.
The particle size of the polymer-enrobed sensitizer is predominantly, i.e.
to an extent of about 70 to 100%, below 20 .mu.m, preferably below 10
.mu.m.
Suitable dye precursors for the recording materials of the invention are in
particular those which will form their dyes on contact with an electron
acceptor by donation of an electron or acceptance of a proton.
Preferred dye precursors are for example the lactones such as, in
particular, crystal violet lactone and the rhodamine- and diaza-rhodamine
lactones, the phthalides, the spirodipyrans such as, in particular, the
spirodibenzopyrans and in particular the fluorans.
Since there are many applications, inter alia the fax process, for which
precursors to black dyes are desirable, particular preference is given for
example to 2-N-phenylamino-3-methyl-6-dialkyl- or -diaryl-aminofluorans.
Of these, 2-N-phenylamino-3-methyl-6-diethylaminofluoran
(diethylaminofluoran for short)
##STR1##
is very particularly notable on account of its high reactivity.
It is of course also possible to use dye precursor mixtures, which may
contain dye precursors from one or more of the classes of compounds
mentioned, in order to obtain a desired color, for example black.
In general, dye precursors which are less reactive and which are not
prematurely developed in the course of the extremely fine grinding in
aqueous medium required for the preparation of the dispersion do not need
to be provided with a protective robe in the thermal papers of the
invention. If, moreover, water-insoluble developers are used, the
protection afforded by the enrobing of the sensitizer is in general
sufficient to prevent even discoloration in the course of the mixed dye
precursor, developer and sensitizer dispersions being applied to the base
paper.
If, however, it is desired, not least for cost reasons, to use dye
precursors and developers which do not meet the abovementioned criteria,
it is advisable to provide the dye precursor by the method described in
DE-A-4 103 966 with a first robe of a water-soluble, nonionic polymer, in
particular polyvinylpyrrolidone (K values according to H. Fikentscher,
Cellulosechemie 13 (1932), 48-64, 71-74, in general of 10 to 100,
preferably to 90, in 1% by weight strength by weight aqueous solution at
25.degree. C.), by grinding in aqueous dispersion and then to enclose it
by polycondensation with a second robe of a crosslinked polymer, in
particular a melamine/formaldehyde resin. For this the directions given in
DE-A-4 103 966 can be followed.
Suitable developers for the thermal papers of the invention are the usual
electron acceptors used for this purpose. There are for example
those based on silicates such as activated and acidic clay, attapulgite,
bentonite, colloidal silicon dioxide and aluminum, magnesium and zinc
silicates,
carboxylic acids such as oxalic, maleic, succinic, tartaric, citric or
stearic acid, benzoic or p-tertbutyl benzoic acid, phthalic acid, gallic
acid and salicylic or substituted salicylic acid such as 3-isopropyl-,
3-cyclohexyl-, 3,5-di-tert-butyl- or 3,5-di(2-methylbenzyl)-salicylic
acid,
phenol derivatives such as 4,4'-isopropylidenediphenol,
-bis(2-chlorophenol), -bis(2,6-dichloro-phenol),-bis
(2,6-dibromophenol),-bis(2,6-dimethylphenol) or -bis(2-tert-butyl phenol),
2,2'-methylene-bis (4-chlorophenol) or -bis(4-methyl-6-butylphenol),
4,4'-sec-butylidenediphenol, 4,4'-cyclohexylidene-diphenol or
-bis(2-methylphenol), 2,2'-dihydroxybiphenyl, 4-tert-butyl- or
4-phenyl-phenol, phenolic resins, .alpha.- or .beta.-naphthol,
methyl 4-hydroxybenzoate, and
salts of these organic acceptors.
Developers based on phenol are preferred for the recording materials of the
invention. 4,4'-Isopropylidenediphenol (bisphenol A) is particularly
preferred.
It is possible for the surface of the developer to be coated with a
polymer. In general, however, this is not necessary, even in the case of
at least partially water soluble compounds, if the dye precursor is
surrounded by a polymer robe.
The heat sensitive recording materials of the invention can be prepared in
a conventional manner. For this a usually about 20 to 70% strength by
weight aqueous dispersion of the (optionally coated) dye precursor, and
about 20 to 70% strength by weight aqueous dispersion of the (optionally
coated) developer and an about 20 to 70% strength by weight aqueous
dispersion of the polymer enrobed sensitizer are either first mixed in a
weight ratio of about 1:4:4 to 1:15:15 or directly added in that weight
ratio to an aqueous solution or dispersion of a binder. Suitable binders
are for example solutions of polyvinyl alcohol and/or binder dispersions
based on styrene/acrylate or styrene/butadiene which may additionally
contain a pigment or a filler such as calcium carbonate, titanium dioxide,
kaolin, a synthetic silicate or a hydrated aluminum oxide. The thermal
dispersions obtained are applied to the base material, usually paper, the
surface of which has in general been glazed, in a layer thickness of about
4 to 7 g/m.sup.2. The paper thus coated is then dried.
The heat-sensitive recording materials of the invention are free of any
visible background staining. The prints have an intensity of black or,
more generally, color which is superior to that obtained with thermal
papers prepared as described in DE-A-4 103 966 without polymer-enrobed
sensitizers. The choice of dye precursor, developer and sensitizer is not
as in US-A-4 520 376 and 4 749 679 restricted to certain substances,
making it possible, in particular, to obtain inexpensive thermal papers
having advantageous application properties.
EXAMPLES
a) Preparation of polymer-enrobed sensitizer
EXAMPLES 1 to 6
In a 4 1 stirred cylindrical vessel with an in-built high performance
dispersing unit (laboratory mixer from Pendraulik, model LD 50, equipped
with a slant toothed disk), a mixture of 600 g of water, 520 g of the
sensitizer X and 106 g of a 20% strength by weight aqueous solution of the
protective colloid
A: a copolymer formed from 236 g 2-acrylamido-2-methylpropanesulfonic acid,
neutralized with 305 g of 20% strength by weight sodium hydroxide
solution, 80 g of methyl acrylate and 4 g of styrene in an aqueous phase
at 85.degree. C. using 0.5 g of potassium peroxodisulphate as initiator
(viscosity in 20% strength by weight aqueous solution at room temperature
and at a shear rate of 489 s.sup.-1 : 1100 mPa.multidot.s), or
B: poly-2-acrylamido-2-methylpropanesulfonic acid (K value 140)
was heated at 3000 rpm to 60.degree. C. and adjusted with 10% strength by
weight formic acid to pH 4.5.
Then y g of a 70% strength by weight aqueous solution of a methylolated,
partially etherified melamine/formaldehyde (M/F) precondensate (see
Example 1 of DE-A-3 512 565) were added at 6000 rpm and T.degree. C. over
52 min while the pH was maintained at 4.5 with an acid.
Then the dispersion was transferred to a propeller-stirred vessel and
heated therein at 80.degree. C. for 2 h with a low degree of shear. After
cooling down to room temperature, the dispersion was neutralized with
triethanolamine.
In each case a milky white dispersion was obtained which was filterable
through a 40 .mu.m filter with virtually no residue.
The particle sizes of the polymer-enrobed sensitizers determined by
microscope and further details concerning these experiments are summarized
in Table 1.
TABLE 1
__________________________________________________________________________
y g of M/F pre-
Protective
T Particle size of polymer-
Example
Sensitizer X
condensate solution
colloid
[.degree.C.]
enrobed sensitizer [.mu.m]
__________________________________________________________________________
1 cetyl alcohol
115 A 60 1-7, max. 12
2 cetyl alcohol
86 A 60 1-7, max. 12
3 cetyl alcohol
58 A 60 1-7, max. 12
4 cetyl alcohol/
115 A 70 1-8, max. 12
stearyl alcohol
(w/w 1:1)
5 stearyl alcohol
86 B 65 1-8, max. 10
6 cetyl alcohol
86 B 60 1-10
__________________________________________________________________________
b) Preparation of enrobed dye precursor
10 g of the dye precursor 2-N-phenylamino-3-methyl-6-diethylaminofluoran
was bead milled with 15 g of water and 1.5 g of a 10% strength by weight
aqueous dispersion of polyvinylpyrrolidone (K 30) to an average particle
size of 1 .mu.m.
Thereafter the dye precursor dispersion thus pretreated was admixed, by
stirring, first with 1.9 g of a mixture of a 70% strength by weight
aqueous solution of melamine-formaldehyde resin (molar ratio melamine:
formaldehyde = from 1:5.6 to 1:6.2) and a 20% strength by weight aqueous
solution of poly-2-acrylamido-2-methylenepropanesulfonic acid (K value
140) in a weight ratio of 1:1 and then with 0.22 g of sodium dihydrogen
phosphate. The pH was then adjusted to 4.2 with formic acid. After
stirring at room temperature for one hour with the addition of 2.5 g of
water, the mixture was stirred at 70.degree. C. for a further 2 h until
fully cured.
The about 35% strength by weight aqueous dispersion of
2-N-phenylamino-3-methyl-6-diethylaminofluoran obtained was free of any
visible coloration. Photometric measurement revealed an intensity value of
0.1 K/S-100.
c) Preparation of thermal papers according to the invention
5 g of a 40% strength by weight aqueous dispersion of the polymer-enrobed
sensitizer obtained under a) (Example 2a: 7.5 g), 1.5 g of a 30% strength
by weight aqueous dispersion of the dye precursor obtained under b) and 4
g of a 50% strength by weight aqueous dispersion of bisphenol A as
developer were added to 3 g of a 5% strength by weight clear aqueous
solution of polyvinyl alcohol (Rhodoviol.RTM. 4/20, Rhone Poulenc)
containing 30% by weight of dispersed calcium carbonate.
The thermal dispersions obtained remained stable, i.e. did not develop any
graying, for a prolonged period.
A thermal dispersion prepared for comparison with unencapsulated sensitizer
rapidly developed a gray color and, what is more, was very difficult to
divide finely.
The thermal dispersions were then applied in a thickness of 7 g/cm.sup.2 to
base paper using a KCC 202 coater from Gockel (Munich).
After drying with hot air at not more than 50.degree. C., the thermal paper
was glazed using a glazing press fitted with a high gloss metal plate by
pressing the thermal paper with its coated side against the metal plate at
room temperature and 100 bar for 1 min.
Development of these heat-sensitive layers was in each case carried out
using an Electronic Systems thermal printer from Neckarsteinach GmbH and
thermal pulses of 1, 2, 3 and 4 ms duration.
The intensity of the blacks obtained was measured with a Datacolor 200
photometer from Leitz in K/S.multidot.100 using the Kubelka-Munk relation
(Kunststoff-Rundschau 17 (1970), 282-291). The reflectance of the uncoated
paper was taken as standard.
The results of these measurements are listed in Table 2.
C denotes, as a comparison for the thermal papers prepared according to the
invention with polymer-enrobed sensitizer (Examples 1 to 4), a thermal
paper prepared in a similar fashion but without the sensitizer and which
produced distinctly worse intensity values.
TABLE 2
______________________________________
Color intensity in
K/S .multidot. 100 on
development under a
thermal pulse of
Example 1 ms 2 ms 3 ms 4 ms
______________________________________
1 1.0 4.7 14.8 22.2
2 1.1 5.6 13.9 21.8
2a 1.5 7.3 16.5 22.5
3 0.9 6.2 15.2 24.1
4 0.7 4.6 13.4 20.4
5 1.5 7.8 15.6 23.1
6 2.9 11.6 21.7 29.3
C 0.3 2.4 10.1 18.0
______________________________________
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